I did this on Psychotic difficulty. Take out all 6 bosses and then go for the Final boss on the Shai-Gen tower. Despite what you may think, he is actually very easy to get to, as long as you don't go through the building to get to him.
1) Equip a FireFly Rocket Launcher and an assault rifle
2) Start out in front of the building, right in front of the front door.
3) Run to the left side of the building. You will see another smaller building with window all over it. Climb that building to the top. You will see a ledge to the Shai-Gen tower. If you jump to this ledge, you just eliminated 50% of enemy encounters. You can go inside the building here and work your way up...
4) Or you can stay on top of that other building. Instead of jumping to that ledge, jump onto the wall over that ledge, you will see some black molding trim sticking out. You must be a LEVEL 4 in agility to use this path to get to the boss.
5) Jump onto that molding trim and hang onto it like your doing a pull up. Now do "pull-up jumps" from black molding to black molding, till you get to a second ledge on the main Shai-gen building. You have now cut your enemy encounters by 75%
6) Run inside and kill everyone using nades or your rifle.
7) Go up the stairs, you should be in a room with large cathedral bell looking things in the middle hanging from the ceiling. Shoot everyone you can in the room. Jump onto a planter and jump to the second level/catwalk going around the room with guys on it, who were shooting down at you. Run around a full 360 degrees and kill all the guys.
8) There should be 3 different stair cases that lead up to the third level/catwalk. Get on the third level and kill everyone. Again do a full 360 degrees around the room. From here jump on top of the closest bell to the third level. Keep hopping on top of bells till your in the middle of the room. You should see a large hole in the ceiling jump into it, and grab hold of ledge and pull your self up.
9) You are now in the main bosses suite/office/home. run around and kill everyone in his suite, about 8-10 guys in all. There is a double staircase leading to the roof. The boss is located in a tower on the roof. Once you exit the stair way immediately turn right. You will see him about 1 story up, shooting down at you along with some of his gun men.
10) Take out your firefly and and lock onto him. Now start unleashed, just keep pulling the trigger till he's dead. It is sometimes easier to jump while your locked onto him and fire, (gives you a clearer shot, hence more damage). All in all it took about 8 rockets on psychotic difficulty.
There is a shortcut to almost every boss. If you need a description on how to take out another boss quickly just send me a PM!